A full-bridge connection, half-bridge connection, or other similar drive circuit is known as a drive circuit for driving a motor or the like (refer, for instance, to JP-A No. 82946/2000 (FIG. 3) hereafter “patent document 1”). A full-bridge connection drive circuit, which is effectively available over a wide power supply voltage range, is disclosed by Patent Document 1.
Conventionally, a half-bridge connection drive circuit 500 shown in FIG. 4 was used. The drive circuit 500 uses a connection node between a high-side MOS transistor Q1 and low-side MOS transistor Q2, for which a drain terminal is connected to the drive circuit power supply voltage VM, as an output terminal OP. These MOS transistors Q1, Q2 are power MOS transistors and have switching functions. A predriver circuit is connected to each gate in order to drive the MOS transistors Q1, Q2.
The low-side MOS transistor Q2 for the drive circuit 500 is grounded. Therefore, when a voltage of 10 V is to be applied to the gate of MOS transistor Q2 during a normal operation, a voltage Vls between 0 V and 10 V is applied to the gate of MOS transistor Q2.
Meanwhile, a source terminal of the high-side MOS transistor Q1 for the drive circuit 500 is the output terminal OP of the drive circuit 500. Therefore, the voltage Vhs to be applied to a gate of MOS transistor Q2 needs to be applied until the sum of the voltage of the output terminal OP (output voltage Vout) and the gate-to-source voltage VGS of MOS transistor Q1 is reached. The output voltage Vout varies from 0 V to the drive circuit power supply voltage VM. Therefore, when gate-to-source voltage VGS having a potential difference of 10 V is required for MOS transistor Q1 during a normal operation, a voltage between 0 V and 25 V needs to be applied to the gate of high-side MOS transistor Q1 if the drive circuit power supply voltage VM is 15 V. If, in this instance, a voltage of 25 V is applied to the gate of MOS transistor Q1 for rapid charging purposes, the gate voltage of MOS transistor Q1 may become instantaneously excessive, thereby damaging the gate.
Further, the predriver circuit shown in FIG. 4 was provided with a circuit that adjusts the voltage of top gate TG while detecting the output voltage Vout of the source terminal of the high-side MOS transistor, that is, the output terminal OP. More specifically, a source reference circuit 505, a protection circuit 506, a clamp circuit 507, and other circuits were provided between the source terminal and gate terminal of MOS transistor Q1. The source reference circuit 505 controls the voltage of the gate terminal with reference to the voltage of the source terminal. The protection circuit 506 is an ESD protection circuit that protects the predriver circuit when static electricity or the like is discharged from the output terminal OP, which is connected to an external device. The clamp circuit 507 protects the source reference circuit 505 and gate terminal when an excessive electrical stress is generated at the output terminal OP. More specifically, the clamp circuit 507 generates a current flow to decrease the potential difference, thereby reducing the electrical stress. In other words, the conventional circuitry was configured to statically provide an operating state.
However, the source reference circuit, protection circuit, and clamp circuit need to be constantly operated to protect the circuitry against a circuit stress generated during operation of the conventional predriver circuit. Thus, electrical power is constantly supplied to the source reference circuit, protection circuit, and clamp circuit. As a result, the power consumption of the conventional predriver circuit is quite high. Therefore, the conventional predriver circuit cannot be used in portable products, which have a limited power supply capacity.
The present invention has been made in view of the above circumstances and provides a predriver circuit so that achieves downsizing and power consumption minimization while preventing a drive's transistor gate from being damaged.